The colonial/Imperial sytems that had always been the norm were weakened after basically constant warfare in europe and the industrial revolution brought large numbers of people from farms in the countryside into the cities to work in factories so living became cramped and in squalor. The Imperial governments greed became more aparent and 2 world wars were the catalyst for countries to shed their colonial rulers. The big empires were broke and could not afford to police them anymore and in most cases did not have the stomache for more warfare to fight the rebellions.

European countries like russia and france had violent over throws of their royal families to try something new. France worked out pretty well while russia didnt.

The 1920s were also the time of the Great Depression which left poor countries bitter toward their rulers and ripe for a dictator to step up with promises to end their suffering and usually blame their ills on one group to unite the whole country behind him. Enter Weimar republic Germany after treaty of Versailles.

The best summaries of Spain's path to Franco's authoritarian leadership are from Paul Preston. Both in The Spanish Civil War and The Spanish Holocaust, Preston lays out a process of "polarization and radicalization" (from Holocaust) that undermined what little tradition of democracy there was in Spain. Both fascism and communism had a good deal of appeal in Spain. However, fascism was much less popular--they secured less than 0.1% of the vote in the 1936 elections that preceded the Spanish Civil War. They were a vocal minority that was relatively well armed, as well as being generally amenable to violence. The reasons for what popularity they had included a fear of "godless communism" (especially given the anticlerical violence that was somewhat common amongst the left even before the war began), a desire for a return to greatness for Spain (especially given the "disaster" of the Spanish-American war), and bizarrely the fear of a "Judeo-Masonic-Bolshevik" international conspiracy to destroy Spain and the Catholic Church. This conspiracy was invented by a number of writers in Spain, particularly a Fr. Tusquets who was heavily influenced by the Protocols of the Elders of Zion.

To get into communism's appeal is much more complicated. The economic situation in Spain resulted in widespread abuse of labor both in the industrial and agricultural sectors. There were also ties to regionalism, especially in Catalonia. Further, discontent with the Spanish Catholic Church's ties to the establishment gave impetus to movements that called for a disentangling of the Church from the state. The communist parties (PCE, POUM and I believe the PSUC as well) were part of the Popular Front that got nearly 40% of the vote in the aforementioned 1936 elections.

I cannot recommend Preston's summaries enough. They are slanted a bit by his pro-Republican bias, but not enough to taint their excellence.

If there is anything else I can clarify or expand upon, please let me know.

Hannah Arendt argued that the rise of totalitarian regimes in the 20th century was at least partially due to the collapse (starting in the Renaissance) of the originally Roman trinity of Tradition, Authority, and Religion. That trinity had, after the fall of Rome, been reasserted by the Church, and the undermining of this trinity, culminating in the philosophy of Karl Marx, led to something of a vacuum in which power manifested itself only through coercion or force.

Some people (those who would become totalitarians) used this vacuum and the wish of people to have some sort (any sort) of authority and tradition to their advantage. Through the use of propaganda and terror among other methods, people like Stalin and Hitler could take control of masses of people longing for an authority.

See Arendt, The Origins of Totalitarianism and Between Past and Future.